Ultra-stable laser from spectral holes in a rare earth-doped crystal at 4K and below.

Ultra-stable lasers are essential components or several high precision measurements experiments and in particular optical atomic clocks. Several approach are currently explored worldwide for making better ultra-stable lasers (whose performance are, for example, the current limitation of the best optical clocks). The talk will focus on one such approach, namely the use of narrow (~kHz large) optical spectral features photo-imprinted in rare-earth-doped crystals at cryogenic temperatures by the technique known as spectral hole burning. The experimental setup developed at SYRTE will be described, and several experiments we have realized there, aiming at assessing the potentiality of the technique and developing tools for targeting ultra-high frequency stability. This involves in particular measuring the sensitivity of the spectral holes to mechanical stress/inertial forces, electro-magnetic perturbations, temperature fluctuations, as well as realizing complex multi-heterodyne probing schemes so as to decrease detection noise (both technical and fundamental). Current achievements demonstrated 1.7E-15 fractional frequency instability at 1s timescale (ie comparable with Fabry-Perot based systems broadly employed in optical clocks worlwide), with prospects for reaching <10E-17 from the effects that have been characterized this far.